This invention relates to a load plus pump control and more particularly to a pump control having a series-parallel bias spring assembly for controlling the pump discharge pressure.
In the control of a variable displacement pump having a movable swash plate for changing the displacement output, a pressure compensated pump control is often provided for urging the swash plate to a multiplicity of positions in response to a plurality of variables of the hydraulic system served by the pump. The pump control often includes a servo valve and a pump discharge cartridge means that interact through various movable elements to provide opposed urging forces on opposed ends of the swash plate for the controlled movement thereof. The urging forces are generally delivered by one or more springs and by fluid pressure at the discharge of the pump and one or more working elements.
One of the problems encountered with pressure compensated pump controls for variable displacement pumps is that the pump output is difficult to control when the pump discharge pressure is extremely low. Thus, it is desirable to maintain the pump discharge pressure above a minimum pressure level in order to have full control of the pump. It is also desirable to maintain the pump discharge pressure at a specified level above the required load pressure. This allows the pump to operate more efficiently over a large range of load pressures. If the difference is too great, excess energy is consumed in driving the pump while if the difference is too small, the pressure losses in the valves and lines between the pump and load might be greater than the "load-plus" difference and the pump efficiency is impaired.